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Pertinent issues affecting Durability of concrete

 
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vijay.kulkarni
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PostPosted: Fri Mar 02, 2012 4:47 pm    Post subject: Pertinent issues affecting Durability of concrete Reply with quote

Dear Friends,


In addition to my comments on crucial issues on concrete durability posted earlier, I am highlighting below some other pertinent issues which need your kind attention.


Use of Advanced Software for Management of QA&QC



Complaints in respect of present-day ready-mixed concrete revolves around three areas — inadequate slump at pour site, lower levels of 7 and 28-day compressive strength and surface cracking.



As some engineers have already pointed out during discussion, the first two problems can be tackled by ensuring that concrete is obtained from an RMC facility having certification — a step that provides an assurance that the RMC supplier has the necessary tools and equipment capable of producing good quality concrete. The second and the most important aspect is about the quality assurance of the product. For this purpose, a robust QA plan should be used. In line with the AASHTO provisions, the RMCMA has already developed Guidelines for QA. The crucial elements of the Quality Plan suggested in the RMCMA guidelines include: (i) continuous monitoring of the properties of all incoming materials (ii) process control in accordance with IS 4926:2003 (iii) continuous monitoring of key fresh and hardened properties of concrete, and (iv) implementing statistical analysis of strength and other properties.



Once the plant and equipment are in order and there is an effective  control on the quality of input materials, then the main problem pertains to the management of the batch-to-variation in the product quality. Since the strength results will only be available at 28 days, some early-warning systems are essential. For this purpose, a technique known as CUSUM (Cumulative summation) technique is quite popular abroad and the same is being used by a few leading RMC producers in India. This technique assists in quick detection of changes in the properties, and indicates when action should be taken to increase the probability of meeting the specifications. In fact, a number of software, (e.g. the ConAd system developed by Ken Day and marketed by Command Alkon) are now available for the effective management of production and quality control. I am aware that presently there are only a few RMC manufacturers who have developed expertise in these areas. But once the consultants start insisting on this the expertise will spread.


I therefore feel that while selecting the RMC supplier for any project, cost should not be the only criteria and the consultants must initially short-list RMC supplier based on their technical skills. The emphasis should first be on selecting a "knowledgeable" RMC supplier, who is capable of implementing the Quality Plan. Proficiency in the CUSUM system can be considered as a bonus. I am sure that such practice will go a long way in mitigating the quality problems.


Non-structural Cracks and Durability Problems


Even when the required slump and strengths have been achieved, there are occasions when surface cracking is either ignored or repaired without proper precautions. This is done without properly understanding the basic reasons for cracking. Assuming that the structural design and detailing are correct, these cracks could be attributed to either plastic shrinkage/plastic settlement or thermal shrinkage or a combination thereof. Quite often, there is a problem in the mixture proportioning itself. Cracking in concrete has today become one of the major points of dispute between the RMC supplier and his clients. I find that there is a lot of misunderstanding about the non-structural cracks. In this context, I would like to draw attention to very interesting and informative publications1, 2.


Non-structural cracks are harmless from structural point-of-view. However, if not treated properly, these cracks can provide direct access to chlorides, sulfates, CO2, etc., creating durability problems at later ages. Further, they mar the aesthetic appearance and hence are not desirable.
India is a tropical country and for an overwhelming majority of our concrete I feel that we should invoke the provisions of Hot Weather Concreting. The ambient temperatures at many locations are higher than 40oC in summer and the relative humilities are also low. Unfortunately, in most of the contracts specific provisions for mitigating the undesirable effects of hot weather concreting are not included.


Why not Specify Concrete Pour Temperature?


Rapid moisture evaporation occurs from concrete surface under hot weather conditions. The rate of evaporation is dependent upon the ambient temperature, concrete temperature, relative humidity, and wind velocity. All these parameters are woven together in the ACI monograph (ACI Committee report 305-R-10), which can be used to estimate the rate of evaporation. The ACI Committee recommends that once the rate of evaporation exceeds 1 kg/m2/h, precautions against plastic shrinkage cracking are necessary.  
Amongst the factors affecting the rate of evaporation, ambient temperature, relative humidity, and wind velocity are dependent upon the vagaries of nature and are beyond our control. However, we can control the concrete temperature at pour site, which should be specified in contracts. IS 7861-Part I -1975 on Extreme Weather Concreting suggests that the temperature of concrete shall be below 40oC at the time of placement, while BS EN 8500-part 2 specifies an upper limit of 35oC. In my opinion, the limit of 40oC is very much on the higher side. This could be similar to the one specified in the British code. In fact the preferable temperature could be around 28-30oC. Of course, achieving concrete temperatures below 30oC will certainly involve additional cost, mainly towards using chilled water or for partly replacing mixing water with ice. However, will it will be advisable to bear the extra cost towards cooling, rather than spending money, time and resources in rectifying the shrinkage cracks at a later stage? Incidentally, in hot weather condition the chemical admixture dosage requirement comes down once we start using chilled water for mixing.


Specify Early Surface Protection and Curing as Separately Payable Items in Contracts
In our usual construction practice we do not care much about providing early surface protection. After placing and compaction, finishing work is done slowly. further,  while the finishing is in progress, the earlier finished portion is left to the mercy of weather. Similar is the fate of curing. The advantages of curing with regards to improved strength gain and enhanced durability need not be overemphasized.


Since both these items are not executed properly, as Adam Neville had suggested, the remedy lies in converting these items as separate items in contracts that are paid for specifically.

Why not Specify Shrinkage in Specifications?
From durability perspective drying and autogenous shrinkages of concrete are important. Now that we have started using HSC/HPC, the latter has assumed importance. However, I am yet to come across a contract in India where the limiting value of concrete shrinkage is specified. It is high time we start this practice, especially for concretes used for bridge decks, pavements, roof slabs, basement raft, etc.
A variety of shrinkage tests is available in the ASTM and other standards. Without going into the controversy, I feel that ASTM C 157 test would be useful in our context. This test method can be used for comparative evaluation of shrinkage potential of different concrete mixes and the one having the lowest value could be short-listed during the initial mix pre-qualification process. Internationally, the accepted limiting values of shrinkages are 0.04% for OPC concrete at 28 days and 0.05% for concrete with supplementary cementtious materials. Interestingly, I find that our own IS 1199-1959 is more or less similar to ASTM C 157 and the same may be used here, to begin with.


Enhancing Durability and Sustainability through increased Use of SCMs



In India, we have limited limestone availability. Even with a moderate GDP growth rate of 7%, the cement requirement by 2017 goes to a whopping 450-500 million t/annum. As per the estimate of our Planning Commission, we do not have sufficient limestone for fulfilling this requirement, unless of course, we disturb whatever is today left of the eco-sensitive areas/forests. Therefore from sustainability perspective, we have no option but to enhance the use of abundantly-available fly ash or  other supplementary cementitious materials (SCMs) in concrete.



There is a plethora of R&D work done worldwide, including India, showing the beneficial effects of fly ash and other SCMs in terms of long-term durability. Yet, there is a lot of apprehension in using SCMs. Yes, one needs to take utmost care in ensuring that good quality SCMs are used by the RMC supplier. There are quick tests to ensure that good quality SCMs are only used. For example, the fly ash wet-sieving test for determining the % retained on 45 micron sieve. In this respect, it is essential to develop agencies which can provide certified and reliable quality SCMs. Also, one needs to develop expertise in designing mixes using SCMs, especially the triple blend (cement + two SCMs) mixes.


Considering the fact that the ternary cement blend system allows high replacement of OPC with fly ash ( say up to 50%) and has a potential to improve the strength and durability properties of concrete, it needs encouragement. Similarly, for large volume pours (raft foundations) and deep lifts, where containment of the heat of hydration is a big problem, why can't we use high-volume fly ash concrete? Possibly, for conditions, where the maximum live load is borne by the structure at a later age, we need to use the provision of either 56-day or 90-day strengths. While doing so, one needs to carry out lab work and establish correlation between 7, 28 and later age strengths. In actual practice, in addition to 56-day or 90-day strength, the 7-day strength should also be specified and monitored so that there is assurance on strength development at later ages.
Yes, the route of using SCMs may seem to be tough, however, we have possibly no option but to master it. It is possibly the only available route to achieve longer durability and enhanced sustainability!


Effect of Fly ash to Sustained High Temperature
The above issue was raised by Jignesh during the discussion. I am answering his query.
With regards to the exposure of concrete containing fly ash to sustained high temperature, no laboratory data is available from India. However, the ACI Committee Report 232.2R-14 states that "fly ash in concrete does not change the mechanical properties of concrete in relation to similar concrete containing only Portland cement when exposed to sustained high temperature conditions ranging from 75-600oC".
A recent paper in ACI Materials Journal by W Khaliq and V K R Kodur has revealed that the reduction in tensile strength in case of fly ash concrete is gradual and almost linear in the 20 to 800°C temperature range. The gradual degradation of strength (without substantial changes) can be attributed to better hydrated cement products and a better matrix in the concrete that results from the fly ash reaction with calcium hydroxide. However, fly ash concrete displayed a significant loss of splitting tensile strength at temperatures beyond 500°C.

References

  1. Non-structural Cracks in  Concrete, Technical report No 22, Concrete Society, U.K.
  2. Early age Thermal Crack  Control in Concrete, CIRIA Report 91, U.K.



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Last edited by vijay.kulkarni on Sat Mar 03, 2012 2:11 am; edited 1 time in total
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PostPosted: Fri Mar 02, 2012 5:49 pm    Post subject: Reply with quote

Dear Sir,

Very useful information and points to think about in this topic.

The temperature of fresh concrete at placement is another sensitive topics you have touched.  

In many critical machine foundation construction, we do specify the temperature of fresh concrete as 23 deg. c.  However, we have many times obtained a request to relax it up to 27 to 30 deg. c.  The machine foundations member sizes are in tune of 3 to 4 m (be it raft, beam or column).  Hence, we treat the same as mass concrete and specify the temperature control.  We also recommend the construction to begin in evening (say 4 pm) as pouring lasts for 30 to 40 hours.  The core of the member will be poured in the night when the temperatures are less compared to day.  

As you have mentioned that we shall specify fresh concrete temperature less than 30 deg. c. for general concreting, do you feel that critical structures which are sensitive to low strains shall specify temperature below 20 deg.  I faintly remember that 19 deg. was recommended for mass concrete.  

Regards,

Jignesh V Chokshi
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PostPosted: Fri Mar 02, 2012 6:21 pm    Post subject: Reply with quote

Dear Sir,

Any material or literature on aggregates being used in various parts of India and its effects on ASR?

Regards,

Jignesh V Chokshi
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narayan_nayak
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PostPosted: Sun Mar 04, 2012 12:52 am    Post subject: Re: Pertinent issues affecting Durability of concrete Reply with quote

I thank Mr. Vijay Kulkarni for informing me about this important E-conference on “Durability of Concrete”.  I also take this opportunity to compliment Dr. N. Subramanian and Mr. Vijay Kulkarni (both conference moderators) and Ms. Alpa Seth, a brain behind this E-conference.
    
         
    
        I have noted valuable issues raised by moderators, Mr. Jignesh V. Chokshi, Mr. Naresh Reddy and Mr. Alok Bhowmick.  Hence  the issues raised by me are other than issues already raised by the above experts.
    
         
    
        There are many issues from my end.  To begin with some of the issues are as noted herein below -
    
         
    
        1. Blended cement - Concrete blended with admixtures like fly ash, GGBS, Metakaolin, micro silica helps in durability and sustainability.
    
         
    
        Since fly ash is available in large quantity in India (180 million tons / annum) alongwith other minerals, we should make it a practice to specify blended cement concrete blended with fly ash, GGBS or metakaolin etc for all grades of concrete and all types of work.
    
         
    
        Bureau of Indian Standards (BIS) unfortunately restricting replacement by fly ash to 35% of the cementatious material.  We should encourage replacement atleast upto 50% (BS code permits upto 55%).  For this we should make all out efforts with the BIS to suitably modify the code.  This will help in durability, workability and sustainability.
    
         
    
        2. We in India produce large quantity of pond ash. (over 125 million tonnes/annum).  Only about 15% is used in fly ash bricks, embankment etc.  As a result disposal of such ash is a great problem and it also poses health hazards.  We should encourage its use as fine aggregate which is beneficial for increased strength and sustainability.  Governmental agencies or power project owners should encourage for lifting such pond ash giving financial incentives.
    
         
    
        3. We produce also significant amount of copper slag basically in 3 parts of India (Gujarat, Jharkhand & Tamilnadu).  We should encourage use of such copper slag for fine aggregate as it generally meets the fine aggregate gradation requirement. Experimental investigations conducted with around 50% replacement of natural sand by copper slag generally gives higher strength (around 10%) and higher density of concrete of around 10%.  However, there is resistance from consultants for lack of codal provision in this connection, though various experiments are conducted in presence of owners as well as consultants.  Such resistance needs to be overcome by training and wide publication on opportunity to use copper slag as fine aggregate.
    
         
    
        4. Curing of concrete is generally far from satisfactory, whether the contractor is well known (branded) contractor or otherwise.  Lack of curing is of further concern for vertical faces, tall structures like high rise buildings, chimneys, cooling towers etc., high performance concrete with micro silica and low water binder ratio.  To overcome this universal problem, it is very necessary to do further research atleast in our country with internal self curing chemical compounds which are just introduced in the Indian market.  If this is found to be very effective, we should encourage the use of such method even at slightly higher cost.
    
         
    
        5. Now a days many structures are very heavily reinforced.  Some of the structures may be having embedded parts.  To avoid honey combing, in such situation, it is advisable to encourage and specify self compacting concrete.
    
         
    
        When embedded parts are used, it is essential to adopt proper concrete pouring sequence otherwise substantial honey combing would occur.  Good concrete pouring sequence indicated herein below in Fig. 1.
    
         
    
       
    
         
    
        Fig. 1

    
         

    
        Sometimes incorrect method of vibrating heaped concrete as shown in Fig. 2 is adopted.  Both these are as a result of lack of training.
    
         
    
       
    
         

    
        Fig. 2

    
         
    
        6.  In India, columns are generally cast after casting "starter".  These starters are cast around 75mm high.  Such starter serves as guide in erection of formwork.  But such starters concrete are rarely vibrated / compacted.  Sometimes even grade of concrete used may be lower than specified for the column.  It may happen when pile cap / raft concrete grade is lower than grade of concrete specified for the column.  In Gammon India Ltd., we recommend to avoid such starters by using appropriate methodology for formwork erection.
    
         
    
        7. Good concrete cover blocks are essential to avoid corrosion of steel.  These cover blocks should be atleast of the same grade of concrete as in structure and they should be formed by proper vibration.  In majority of the sites, cover blocks are made at site by producing concrete on 10/7 mixers and without any vibration.  Grade of concrete of cover blocks may generally be lower than used in structure.  Such poor cover blocks provide easy access for corrosion to occur.  To avoid such situation, some consultants recommend plastic cover blocks. I do not favor such cover blocks because of compatibility problem between plastic material and concrete.  I recommend the use of precast concrete cover blocks of M60 grade duly vibrated, if we can not manufacture cover blocks at site using vibrating table.
    
         
    
        8. Joints are sources of leakage and easy access for corrosion.  In Gammon India Ltd., we recommend practically seamless joints by suitable arrangement in concrete formwork.
    
         
    
        9.  As a result of lack of tranined manpower, long binding wires, longer than essential are used in reinforcement tying, sometimes leaving the free end of the binding wire in the cover zone.  This will aid early corrosion.
    
         
    
        10.  In India atleast in 95% of the cases, concrete above the pile cut-off is removed by percussion method (See Fig. 3 & 4).  In recent years some have started crushing the concrete by using hydraulic machine which gives the faster progress.  Both the methods are not desirable as they generate fine cracks in concrete below the cut-off which allow easy access to the water.  We recommend removal of concrete above the cut off in the green stage itself and compact concrete at cut-off level after removal which is cost and time effective in addition to enabling producing durable pile concrete.  This is now specified in IRC 78 but still majority as noted earlier use percussion method for removal of concrete above the cut-off.
    
         
    
         
    
       
    
                           Fig. 3                                                           Fig. 4
    
         
    
        11. Even today, many consultants / owners (around 50%) specify L-bend to reinforcement cage (See Fig. 5).  This is not desirable.  Such L - bends obstruct proper flushing of bore hole bottom and prevents tremie to reach pile tip bottom particularly in small diameter pile which is essential for good quality concrete.  If tremie does not reach pile tip bottom, concreteing by displacement method is not achieved which is essential for durable concrete.
    
         
    
       

    
        Fig. 5

    
         
    
        12.  In India, on number of occasions under-reamed piles are adopted.  Earlier concreting was done by head load method and concreting done slowly but with use of concrete pump speed of concreting is very fast.  If at under reamed location, concreting is done very fast, only part under ream will be formed as shown in Fig. 6.  Hence, it is essential to do concreting very slowly at under ream location otherwise full capacity of under ream will not be achieved.
    
         
    
       

    
         

    
        Fig. 6

    
         

    
        13. I am aware in one of the high rise building projects, 3.5 m thick raft with M50 grade of concrete was cast in 1 pour without bothering about temperature control while producing concrete or thereafter.  Further cementatious content used for the concrete was OPC (53 grade) 460 kg/m3 and micro silica of 100 kg/m3.  As a result temperature of 860 C was recorded at the center of the raft.
    
         
    
        One could have easily produce M50 grade concrete with OPC cement and fly ash without the use of micro silica with the total cementatious content of around 430 kg/m3 (with OPC and fly ash in the ration of 65% :35%).  Further cementatious content could have been reduced further by 40 mm down coarse aggregate.  Also no attempt was made to lay the concrete in layers.
    
         
    
        Even today still many people believe that more the cement better is the concrete rather than minimum (optimum) requirement is the best and blending is an improvement.  Similarly it is the fad to use micro silica even when it is not essential.  Today, atleast upto M60 grade of concrete, one can easily eliminate micro silica unless it is to be used for some other specific reason.
    
         
    
        14. Lack of traning is the main cause for poor quality of execution as brought out by Mr. Alok Bhowmick.  To achieve quality, durability, cost reduction, improved cycle time, wastage control, innovative approach to execution and new concepts for sustainable growth, “On Hand Field Training” by experienced technocrat manager is essential.  Such training is far more effective than class room training.  Keeping this in mind, I am planning to take such task in the interest of achieving durable, economical and sustainable construction.
    
         
    
        Regards,
    
         
    
         
    
        N.V. Nayak
    
        Managing Director
    
        Gammon Realty Ltd.
         
    
         

     
vijay.kulkarni wrote:
Dear Friends,


In addition to my comments on crucial issues on concrete durability posted earlier, I am highlighting below some other pertinent issues which need your kind attention.


Use of Advanced Software for Management of QA&QC



Complaints in respect of present-day ready-mixed concrete revolves around three areas — inadequate slump at pour site, lower levels of 7 and 28-day compressive strength and surface cracking.



As some engineers have already pointed out during discussion, the first two problems can be tackled by ensuring that concrete is obtained from an RMC facility having certification — a step that provides an assurance that the RMC supplier has the necessary tools and equipment capable of producing good quality concrete. The second and the most important aspect is about the quality assurance of the product. For this purpose, a robust QA plan should be used. In line with the AASHTO provisions, the RMCMA has already developed Guidelines for QA. The crucial elements of the Quality Plan suggested in the RMCMA guidelines include: (i) continuous monitoring of the properties of all incoming materials (ii) process control in accordance with IS 4926:2003 (iii) continuous monitoring of key fresh and hardened properties of concrete, and (iv) implementing statistical analysis of strength and other properties.



Once the plant and equipment are in order and there is an effective  control on the quality of input materials, then the main problem pertains to the management of the batch-to-variation in the product quality. Since the strength results will only be available at 28 days, some early-warning systems are essential. For this purpose, a technique known as CUSUM (Cumulative summation) technique is quite popular abroad and the same is being used by a few leading RMC producers in India. This technique assists in quick detection of changes in the properties, and indicates when action should be taken to increase the probability of meeting the specifications. In fact, a number of software, (e.g. the ConAd system developed by Ken Day and marketed by Command Alkon) are now available for the effective management of production and quality control. I am aware that presently there are only a few RMC manufacturers who have developed expertise in these areas. But once the consultants start insisting on this the expertise will spread.


I therefore feel that while selecting the RMC supplier for any project, cost should not be the only criteria and the consultants must initially short-list RMC supplier based on their technical skills. The emphasis should first be on selecting a "knowledgeable" RMC supplier, who is capable of implementing the Quality Plan. Proficiency in the CUSUM system can be considered as a bonus. I am sure that such practice will go a long way in mitigating the quality problems.


Non-structural Cracks and Durability Problems


Even when the required slump and strengths have been achieved, there are occasions when surface cracking is either ignored or repaired without proper precautions. This is done without properly understanding the basic reasons for cracking. Assuming that the structural design and detailing are correct, these cracks could be attributed to either plastic shrinkage/plastic settlement or thermal shrinkage or a combination thereof. Quite often, there is a problem in the mixture proportioning itself. Cracking in concrete has today become one of the major points of dispute between the RMC supplier and his clients. I find that there is a lot of misunderstanding about the non-structural cracks. In this context, I would like to draw attention to very interesting and informative publications1, 2.


Non-structural cracks are harmless from structural point-of-view. However, if not treated properly, these cracks can provide direct access to chlorides, sulfates, CO2, etc., creating durability problems at later ages. Further, they mar the aesthetic appearance and hence are not desirable.
India is a tropical country and for an overwhelming majority of our concrete I feel that we should invoke the provisions of Hot Weather Concreting. The ambient temperatures at many locations are higher than 40oC in summer and the relative humilities are also low. Unfortunately, in most of the contracts specific provisions for mitigating the undesirable effects of hot weather concreting are not included.


Why not Specify Concrete Pour Temperature?


Rapid moisture evaporation occurs from concrete surface under hot weather conditions. The rate of evaporation is dependent upon the ambient temperature, concrete temperature, relative humidity, and wind velocity. All these parameters are woven together in the ACI monograph (ACI Committee report 305-R-10), which can be used to estimate the rate of evaporation. The ACI Committee recommends that once the rate of evaporation exceeds 1 kg/m2/h, precautions against plastic shrinkage cracking are necessary.  
Amongst the factors affecting the rate of evaporation, ambient temperature, relative humidity, and wind velocity are dependent upon the vagaries of nature and are beyond our control. However, we can control the concrete temperature at pour site, which should be specified in contracts. IS 7861-Part I -1975 on Extreme Weather Concreting suggests that the temperature of concrete shall be below 40oC at the time of placement, while BS EN 8500-part 2 specifies an upper limit of 35oC. In my opinion, the limit of 40oC is very much on the higher side. This could be similar to the one specified in the British code. In fact the preferable temperature could be around 28-30oC. Of course, achieving concrete temperatures below 30oC will certainly involve additional cost, mainly towards using chilled water or for partly replacing mixing water with ice. However, will it will be advisable to bear the extra cost towards cooling, rather than spending money, time and resources in rectifying the shrinkage cracks at a later stage? Incidentally, in hot weather condition the chemical admixture dosage requirement comes down once we start using chilled water for mixing.


Specify Early Surface Protection and Curing as Separately Payable Items in Contracts
In our usual construction practice we do not care much about providing early surface protection. After placing and compaction, finishing work is done slowly. further,  while the finishing is in progress, the earlier finished portion is left to the mercy of weather. Similar is the fate of curing. The advantages of curing with regards to improved strength gain and enhanced durability need not be overemphasized.


Since both these items are not executed properly, as Adam Neville had suggested, the remedy lies in converting these items as separate items in contracts that are paid for specifically.

Why not Specify Shrinkage in Specifications?
From durability perspective drying and autogenous shrinkages of concrete are important. Now that we have started using HSC/HPC, the latter has assumed importance. However, I am yet to come across a contract in India where the limiting value of concrete shrinkage is specified. It is high time we start this practice, especially for concretes used for bridge decks, pavements, roof slabs, basement raft, etc.
A variety of shrinkage tests is available in the ASTM and other standards. Without going into the controversy, I feel that ASTM C 157 test would be useful in our context. This test method can be used for comparative evaluation of shrinkage potential of different concrete mixes and the one having the lowest value could be short-listed during the initial mix pre-qualification process. Internationally, the accepted limiting values of shrinkages are 0.04% for OPC concrete at 28 days and 0.05% for concrete with supplementary cementtious materials. Interestingly, I find that our own IS 1199-1959 is more or less similar to ASTM C 157 and the same may be used here, to begin with.


Enhancing Durability and Sustainability through increased Use of SCMs



In India, we have limited limestone availability. Even with a moderate GDP growth rate of 7%, the cement requirement by 2017 goes to a whopping 450-500 million t/annum. As per the estimate of our Planning Commission, we do not have sufficient limestone for fulfilling this requirement, unless of course, we disturb whatever is today left of the eco-sensitive areas/forests. Therefore from sustainability perspective, we have no option but to enhance the use of abundantly-available fly ash or  other supplementary cementitious materials (SCMs) in concrete.



There is a plethora of R&D work done worldwide, including India, showing the beneficial effects of fly ash and other SCMs in terms of long-term durability. Yet, there is a lot of apprehension in using SCMs. Yes, one needs to take utmost care in ensuring that good quality SCMs are used by the RMC supplier. There are quick tests to ensure that good quality SCMs are only used. For example, the fly ash wet-sieving test for determining the % retained on 45 micron sieve. In this respect, it is essential to develop agencies which can provide certified and reliable quality SCMs. Also, one needs to develop expertise in designing mixes using SCMs, especially the triple blend (cement + two SCMs) mixes.


Considering the fact that the ternary cement blend system allows high replacement of OPC with fly ash ( say up to 50%) and has a potential to improve the strength and durability properties of concrete, it needs encouragement. Similarly, for large volume pours (raft foundations) and deep lifts, where containment of the heat of hydration is a big problem, why can't we use high-volume fly ash concrete? Possibly, for conditions, where the maximum live load is borne by the structure at a later age, we need to use the provision of either 56-day or 90-day strengths. While doing so, one needs to carry out lab work and establish correlation between 7, 28 and later age strengths. In actual practice, in addition to 56-day or 90-day strength, the 7-day strength should also be specified and monitored so that there is assurance on strength development at later ages.
Yes, the route of using SCMs may seem to be tough, however, we have possibly no option but to master it. It is possibly the only available route to achieve longer durability and enhanced sustainability!


Effect of Fly ash to Sustained High Temperature
The above issue was raised by Jignesh during the discussion. I am answering his query.
With regards to the exposure of concrete containing fly ash to sustained high temperature, no laboratory data is available from India. However, the ACI Committee Report 232.2R-14 states that "fly ash in concrete does not change the mechanical properties of concrete in relation to similar concrete containing only Portland cement when exposed to sustained high temperature conditions ranging from 75-600oC".
A recent paper in ACI Materials Journal by W Khaliq and V K R Kodur has revealed that the reduction in tensile strength in case of fly ash concrete is gradual and almost linear in the 20 to 800°C temperature range. The gradual degradation of strength (without substantial changes) can be attributed to better hydrated cement products and a better matrix in the concrete that results from the fly ash reaction with calcium hydroxide. However, fly ash concrete displayed a significant loss of splitting tensile strength at temperatures beyond 500°C.

References

  1. Non-structural Cracks in  Concrete, Technical report No 22, Concrete Society, U.K.
  2. Early age Thermal Crack  Control in Concrete, CIRIA Report 91, U.K.
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PostPosted: Sun Mar 04, 2012 7:02 am    Post subject: Reply with quote

Dear Members,

Increased use of Fly Ash is strongly desired by many people.  Various national codes limit the use of fly ash upto 35 % to 55%.  

The production of Fly ash as by product of coal based power plants is in abundance.  Some posts referred the quantum of the same in terms of Mill. tons per annum.  

Suppose we had no coal based power plant and all power plants would be hydro, Gas based, solar or nuclear power plants, would addition of Fly ash be recommended and debated?  Use of fly ash in building construction is compulsion or an alternative?

Did ancient structures used fly ash and if yes, what was the source?  

Regards,

Jignesh V Chokshi
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JVCSNL
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Joined: 26 Jan 2003
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PostPosted: Sun Mar 04, 2012 7:39 am    Post subject: Reply with quote

Dear Mr. Nayak,

Thanks for raising many other topics which were not touched so far.

You have touched a critical issue of pile cut off method.  As you have rightly said, the pile cut off shall be achieved during green phase to avoid potential cracks in concrete.

However, many times the foundations are required at deeper level (say 8 to 10m ) below natural ground level. Hence, the pile cut off will also require at deeper level.  The problem becomes more serious when the foundations of structures are required at different level due to various requirements.  In such cases in order to move the piliing rig, we can not excavate the ground before piling is completed.  

What would be the pile cut off method in such situations? Any alternative you would prefer to be considered and specified in design drawings?

Regards,

Jignesh V Chokshi
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narayan_nayak
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Joined: 02 Mar 2012
Posts: 9

PostPosted: Mon Mar 05, 2012 4:15 am    Post subject: Pertinent issues affecting Durability of concrete Reply with quote

Dear Jignesh Chokshi,

Thank you for raising very relevant question about removing the green concrete when the pile cut off is very deep below ground level.

Even in such case, concrete above cut off is to be removed in green stage only. For this, I have developed a very simple tool and I have used this tool very successfully even when cut of is 5 to 6 m below the ground level. Similarly. I have developed a very simple tool to compact the concrete after removal in green stage. Such compaction is absolutely essential to see that there are no air voids or honey comb are left as the slump of the concrete may likely to become quite low at the time of removal of green concrete.

Both these devices are illustrated in my book “Foundation Design Manual – 6th Edn.” (published in Feb.2012 by M/s Dhanpatrai Publications, Delhi). These devices/tools can be used even when the cut off is 8 to 10 m below ground level.

Further, with deep cut off when piles are in small group I recommend leaving the temporary liners in position even after removal of the concrete in green stage till the excavation. These liners are removed after the excavation is done to the required level. But when pile group is very large like in case of big raft foundation, it may not be advisable to leave liners till the excavation. In such a situation after removal of green concrete upto cut off level, pile bore is filled by sand, then the liner is withdrawn upto cut off level. For easy removal of such liners, I have developed a simple system wherein separate liner is provided above the cut off level with “ear-key” type locking arrangement for easy removal of the liner above the cut off. This has also been explained in my above mentioned book.

Trust., this clarifies fully the question raised by you. If you need any further clarification, you may contact me on 09821116010.

Thanks and regards,

NV Nayak



From: JVCSNL [mailto:forum@sefindia.org]
Sent: Sunday, March 04, 2012 1:09 PM
To: econf@sefindia.org
Subject: [ECONF] Re: Pertinent issues affecting Durability of concrete



Dear Mr. Naik,

Thanks for raising many other topics which were not touched so far.

You have touched a critical issue of pile cut off method. As you have rightly said, the pile cut off shall be achieved during green phase to avoid potential cracks in concrete.

However, many times the foundations are required at deeper level (say 8 to 10m ) below natural ground level. Hence, the pile cut off will also require at deeper level. The problem becomes more serious when the foundations of structures are required at different level due to various requirements. In such cases in order to move the piliing rig, we can not excavate the ground before piling is completed.

What would be the pile cut off method in such situations? Any alternative you would prefer to be considered and specified in design drawings?

Regards,

Jignesh V Chokshi







Think Green & Live green.
This e-mail is confidential and it is intended only for the addressees. Any review, dissemination, distribution, or copying of this message by persons or entities other than the intended recipient is prohibited. If you have received this e-mail in error, kindly notify us immediately by telephone or e-mail and delete the message from your system. The sender does not accept liability for any errors or omissions in the contents of this message which may arise as a result of the e-mail transmission.""

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narayan_nayak
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Joined: 02 Mar 2012
Posts: 9

PostPosted: Mon Mar 05, 2012 6:25 am    Post subject: Pertinent issues affecting Durability of concrete Reply with quote

Dear Jignesh Chokshi,

Thank you for raising very relevant question about removing the green concrete when the pile cut off is very deep below ground level.

Even in such case, concrete above cut off is to be removed in green stage only. For this, I have developed a very simple tool and I have used this tool very successfully even when cut of is 5 to 6 m below the ground level. Similarly. I have developed a very simple tool to compact the concrete after removal in green stage. Such compaction is absolutely essential to see that there are no air voids or honey comb are left as the slump of the concrete may likely to become quite low at the time of removal of green concrete.

Both these devices are illustrated in my book “Foundation Design Manual – 6th Edn.” (published in Feb.2012 by M/s Dhanpatrai Publications, Delhi). These devices/tools can be used even when the cut off is 8 to 10 m below ground level.

Further, with deep cut off when piles are in small group I recommend leaving the temporary liners in position even after removal of the concrete in green stage till the excavation. These liners are removed after the excavation is done to the required level. But when pile group is very large like in case of big raft foundation, it may not be advisable to leave liners till the excavation. In such a situation after removal of green concrete upto cut off level, pile bore is filled by sand, then the liner is withdrawn upto cut off level. For easy removal of such liners, I have developed a simple system wherein separate liner is provided above the cut off level with “ear-key” type locking arrangement for easy removal of the liner above the cut off. This has also been explained in my above mentioned book.

Trust., this clarifies fully the question raised by you. If you need any further clarification, you may contact me on 09821116010.

Thanks and regards,

NV Nayak



From: JVCSNL [mailto:forum@sefindia.org] ([mailto:forum@sefindia.org])
Sent: Sunday, March 04, 2012 1:09 PM
To: econf@sefindia.org (econf@sefindia.org)
Subject: [ECONF] Re: Pertinent issues affecting Durability of concrete



Dear Mr. Naik,

Thanks for raising many other topics which were not touched so far.

You have touched a critical issue of pile cut off method. As you have rightly said, the pile cut off shall be achieved during green phase to avoid potential cracks in concrete.

However, many times the foundations are required at deeper level (say 8 to 10m ) below natural ground level. Hence, the pile cut off will also require at deeper level. The problem becomes more serious when the foundations of structures are required at different level due to various requirements. In such cases in order to move the piliing rig, we can not excavate the ground before piling is completed.

What would be the pile cut off method in such situations? Any alternative you would prefer to be considered and specified in design drawings?

Regards,

Jignesh V Chokshi







Think Green & Live green.
This e-mail is confidential and it is intended only for the addressees. Any review, dissemination, distribution, or copying of this message by persons or entities other than the intended recipient is prohibited. If you have received this e-mail in error, kindly notify us immediately by telephone or e-mail and delete the message from your system. The sender does not accept liability for any errors or omissions in the contents of this message which may arise as a result of the e-mail transmission.""

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JVCSNL
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Posts: 158

PostPosted: Mon Mar 05, 2012 2:20 pm    Post subject: Reply with quote

Dear Mr. Nayak,

Thanks a lot for your detailed explanation on an important aspect of pile cut off level.

I will certainly study the methods given in the latest edition of your book. I will contact in case I need further details.

Thanks and Regards,

Jignesh V Chokshi
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